How should user registration happen in SRP? Is there a standard or paper describing it?
As mentioned in the comments this is a standard problem not unique to SRP and not really about the cryptography of SRP. So this question would probably be better posted on 'security.stackexchange.com' as it is more of a generic problem.
Fundamentally setting a shared secret like a regular password or a password verifier over a public network has its challenges. For most applications the standard techniques used to reset a regular password can be reused in the setting of the password verifier.
The only choices you have is whether the salt of the user is generated by the client or the server. It makes sense for the server to do this as it can ensure that the salt has high entropy and is unique to the user.